Loans Held for Investment - Aging (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	$ 82,322	$ 55,823
Current Loans	7,262,928	7,325,577
Total loans	7,345,250	7,381,400
Accruing Loans Past Due 90 Days or More	144	3
Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	46,334	31,935
Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	23,343	7,918
Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	12,645	15,970
Prime Lending | U.S. Government Agencies
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Accruing Loans Past Due 90 Days or More	101,200	102,700
Unpaid principal balance loans past due 90 days or more	102,400	104,000
Commercial Real Estate
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	3,062,042	3,000,523
Commercial Real Estate | Non-owner occupied
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	23,459	6,852
Current Loans	1,721,265	1,702,500
Total loans	1,744,724	1,709,352
Commercial Real Estate | Non-owner occupied | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	6,488	4,062
Commercial Real Estate | Non-owner occupied | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	16,772
Commercial Real Estate | Non-owner occupied | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	199	2,790
Commercial Real Estate | Owner occupied
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	18,237	5,958
Current Loans	1,299,081	1,285,213
Total loans	1,317,318	1,291,171
Commercial Real Estate | Owner occupied | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	12,152	1,813
Commercial Real Estate | Owner occupied | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	2,685	880
Commercial Real Estate | Owner occupied | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	3,400	3,265
Commercial | Commercial And Industrial
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	10,633	11,097
Current Loans	2,091,335	2,014,623
Total loans	2,101,968	2,025,720
Accruing Loans Past Due 90 Days or More	47	3
Commercial | Commercial And Industrial | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	6,740	5,967
Commercial | Commercial And Industrial | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	1,125	1,735
Commercial | Commercial And Industrial | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	2,768	3,395
Construction and land development
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	4,056	9,407
Current Loans	951,117	931,157
Total loans	955,173	940,564
Accruing Loans Past Due 90 Days or More	97
Construction and land development | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	3,955	7,580
Construction and land development | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	3	1,827
Construction and land development | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	98
1 - 4 family residential
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	25,328	22,020
Current Loans	651,302	769,000
Total loans	676,630	791,020
1 - 4 family residential | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	16,832	12,058
1 - 4 family residential | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	2,600	3,442
1 - 4 family residential | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	5,896	6,520
Consumer
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	609	489
Current Loans	42,578	46,557
Total loans	43,187	47,046
Consumer | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	167	455
Consumer | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	158	34
Consumer | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	284
Broker-dealer
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Current Loans	506,250	576,527
Total loans	$ 506,250	$ 576,527